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The Optimization of the Multi-Atmospheric Ar-Xe Laser

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The Optimization of the Multi-Atmospheric Ar-Xe Laser 250 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 34, NO. 2, FEBRUARY 1998 The Optimization of the Multi-Atmospheric Ar–Xe Laser S. W. A. Gielkens, W. J. Witteman, V. N. Tskhai, and P. J. M. Peters Abstract— The quasi-steady-state conditions of the multi- make this laser also a competitor for the well-developed CO atmospheric e-beam sustained Ar–Xe laser are investigated. It laser. The advantages of this new atomic laser are the absence is observed that the duration of the stationary period depends of dissociation and the regeneration of the laser gas, and on the e-beam current, discharge power deposition, and gas pressure. The laser efficiency can be as high as 8%. Beyond the the much shorter wavelength of about m compared to stationary period the efficiency drops. The pulse energy with CO lasers. optimum efficiency depends strongly on the gas pressure. The The full exploration of the new system requires a detailed maximum discharge efficiency of 5%–6% is at high pressure study of its parameters and its kinetic chain of the inversion not sensitive to the input power. The best results are obtained production. The understanding of the laser and the quantitative for 4 bar with a discharge input power of 8 MW/. The pulse duration with corresponding output energies is 12 "s with information on the kinetics can then be used for the design of 10 J/ and 16 "s with 16 J/ for e-beam currents of 0.4 and an optimized system. From an experimental point of view, it 0.9 A/cmPY respectively. An analysis of the quasi-steady-state is attractive to study this atomic xenon laser by means of an conditions that include the effects of electron collision mixing -beam sustained device because this technique has shown so and atomic quenching is presented. The effects of output power far to be most productive and efficient for radiation produc- saturation by the fractional ionization and atomic collisions are in agreement with the observations. The analysis clarifies the tion. Moreover, it allows to follow the effects of discharge optimum performance conditions. parameters and gas composition more or less independently and in this way unravels kinetic processes that otherwise may Index Terms— Electric discharge pumping, electron beam pumping, gas lasers, lasers, laser thermal factors, power lasers, be strongly mixed. For instance, the -beam produces a stable pulsed lasers. homogeneous plasma, independent on the gas pressure, so that the study of gas density effects is not hampered by plasma instabilities that in self-sustained discharges are automatically introduced by the increase of the gas pressure. In principle, I. INTRODUCTION the -beam sustained discharge allows to follow more or less HE STUDY of the laser transitions between the and independently the effects of discharge current, e-beam current, T bands of xenon [1]–[7] is of considerable interest for and gas pressure. several reasons. First of all, it is from a scientific point of In our previous work [7], we used a short e-beam pulse view remarkable that these lasing infrared transitions can be of only 1.2 s and a much longer discharge pulse. These very efficient up to 8% depending on discharge conditions experiments showed the fast drop of the output power after and that pulse energies up to 15 J with power densities of termination of the -beam. This work clarified the necessity several MW can be obtained [7]. Secondly, the apparent of simultaneous operation of discharge and -beam. Further- favorable kinetic chain of this laser process based on three- more, the experiments revealed the more or less quadratic body collisions challenged the development of CW systems dependence of the optimum input power on the gas pressure. with output powers in the order of watts [8], [9]. This became The experiments also brought forward the question to what successful with RF excitation of a mixture at 90 torr in narrow extent we are dealing with the quasi-steady state during the waveguide structures where output power densities of about simultaneous presence of the pulses and what the saturation 0.27 W/cm were obtained, which is two or three orders of mechanisms are. We particularly want to have more insight magnitude higher than what was previously known for low- into the quenching effects of electrons and atoms. To study pressure atomic discharge Xe lasers. This breakthrough in the these questions, we reconstructed our system to have simulta- gas laser development of obtaining high power combined with neous pulses for the -beam and sustainer of about 20 s. For the typical high optical quality opens the gate to many new this device, we observed the output waveforms as a function promising applications, e.g., the field of remote sensing and of -beam current, discharge current, and gas pressure. A communications. The high efficiency and high output power kinetic model is developed to get more insight into the kinetic processes. Manuscript received July 17, 1997; revised October 9, 1997. These investi- gations in the program of the Foundation for Fundamental Research on Matter were supported in part by the Netherlands Technology Foundation (STW). II. EXPERIMENTAL SETUP The authors are with the Department of Applied Physics, University of Twente, 7500 AE Enschede, The Netherlands. The electron gun is based on a plasma cathode and is Publisher Item Identifier S 0018-9197(98)01097-5. described elsewhere [10]. The -beam current density after 0018–9197/98$10.00 1998 IEEE GIELKENS et al.: OPTIMIZATION OF THE MULTI-ATMOSPHERIC Ar–Xe LASER 251 passing the 15- m-thick Ti foil was varied between 0.25 and 0.9 A/cm . In our experiments, the accelerator voltage was kept constant at 185 kV. The discharge circuit consists of three capacitors with a capacitance of 30 F each and two inductors each of 400 nH to provide for a more or less rectangular shape with a duration of about 20 s. The discharge is switched on by the -beam. The discharge is maintained between the foil and (a) an additional electrode. To avoid sputtering, we used the foil as anode. When this foil was used as cathode the sputtering resulted already in foil rupture at a current of 20 kA. The resonator consists of a flat totally reflecting Cu mirror and a plan-parallel ZnSe output coupler with a reflectance of 50%. These mirrors are separated by 90 cm. The distance between the electrodes is 2 cm and the cross section of the -beam is 3 53 cm . The laser extraction volume is 0.31 and the (b) base vacuum in the laser chamber was 5 10 bar. High- purity argon (99.9990%) and xenon (99.990%) were used. The beam and discharge current were measured by Rogowski coils. The accelerating and discharge voltage were measured by resistive voltage dividers. The contribution of an inductive element to the measured voltage appeared to be negligible. By multiplication of the measured discharge current and voltage the input power of the discharge was calculated. The power deposition by the -beam is calculated from stopping power (c) data [11]. The laser oscillates on several transitions between t the and levels of Xe. The temporal profile of the total Fig. 1. Temporal profiles of the (a) beam current density em, (b) dis- charge current sdis, and (c) laser output power out. output power is measured by a fast uncooled InAs photodiode (EG&G J12-18c) in combination with a CdTe window that transmits all laser lines but blocks visible radiation. The total product of the chosen -beam current and gas pressure because output energy is detected by a pyroelectric joulemeter (Gentec at higher input powers of the discharge when the stationary ED 500). By comparison of the measured energy with the period is short, it is observed that a substantial part of the measured waveform detected by the photodiode, the amplitude stationary output power is already present during the build-up of this diode signal is converted into units of power. time of the -beam. For each picture, the total input power can be inferred from Fig. 2. In this figure for each experimental condition the corresponding beam input power and discharge III. EXPERIMENTAL OBSERVATIONS power is plotted. The results of the stationary time as a function The typical behavior of the pulsed experiment is the ap- of the discharge power are shown in Fig. 3(a) and (b) for - pearance of the output pulse shortly after the onset of the beam current densities of 0.4 and 0.9 A/cm respectively. discharge pulse, followed by a quasi-steady-state regime where It is clearly seen that the stationary time strongly depends the -beam current, discharge current, and output power are on pressure. Although the experimental data are somewhat more or less constant and finally the region with the premature scattered owing to experimental fluctuations, the stationary fall-off of the output pulse whereas the discharge and -beam time is roughly inversely proportional to the discharge power. pulses are still present (see Fig. 1). The stationary period is Beyond this stationary regime, the output power and laser then determined by the time during which the output power efficiency decrease. The output power in the stationary regime is more or less constant. At the end of this period, we always as a function of discharge power is plotted in Fig. 4(a) and observe a continuous fast drop of the output. The present (b) for -beam current densities of 0.4 and 0.9 A/cm paper only considers the total lasing potential of the two bands respectively. The general behavior of increase of the output by investigation of the multiwave mode.
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